A sort of fungus. The word is a corruption of Puck or Pouk ball, anciently called Puck-fist. The Irish name is Pooka-foot. (Saxon, Pulker-fist, a toadstool.) Shakespeare alludes to this superstition when Prospero summons amongst his elves
You whose pastime Is to make midnight mushrooms.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894